Mexican authorities detained more than 500 undocumented migrants over the weekend, mostly Central Americans trying to reach the United States, many of them risking their lives, officials said.

Many of the migrants, who were detained in six different operations, were being trafficked by smugglers in unventilated trucks with no food or water.

More than 200 others were found crammed into six "safe houses" used by criminal gangs in the state of Tamaulipas, which borders the United States.

They had paid their traffickers up to $4,000. More than 100 of the migrants in that group were minors.

The detentions come as immigration across the southern US border is at the center of a political firestorm in Washington.

The US Congress launched debate Monday on a proposal from President Donald Trump to grant citizenship to 1.8 million immigrants brought to the United States as children -- a concession to opposition Democrats -- in exchange for tough cutbacks on immigration and $25 billion for tighter border security, including his planned wall.

Mexico's National Migration Institute said Saturday that 100 undocumented Guatemalans and Hondurans including 41 minors were found in two buses in the western state of Jalisco, after paying a smuggler between $5,000 and $7,000 each.

In another operation, 53 Guatemalans including 13 minors were found in an unventilated truck in the violent eastern state of Veracruz.

Authorities arrested a total of nine people on human trafficking charges in the two operations.

On Sunday, authorities said they arrested a group of 228 Central American migrants and one Mexican in Tamaulipas, after receiving an anonymous tip-off.

And on Monday, they announced three more operations that led to the detention of 40 Hondurans in an overstuffed and unventilated truck in Veracruz, 43 Central Americans in Tamaulipas and 41 in the state of Tabasco.

The operations come a week after Mexico detained more than 300 Central American men, women and children being smuggled to the US without water or food.

Amnesty International recently reported that Mexico deports thousands of Central Americans without taking into account their potential rights to treatment as refugees, such as when they are persecuted by brutally violent gangs in their home countries.